Abstract Aberrant activation of the PI 3-K/Akt pathway is directly involved in the tumorigenesis of breast cancer. Whereas extensive studies have shown that Akt enhances tumor cell survival and proliferation, limited information is known regarding the functions of Akt in cancer metastasis, which contributes to majority of cancer mortality. Studies from our laboratory have shown that Akt1 and Akt2 have opposing effects on breast cancer cell motility. In this project, I propose to test the hypothesis that Akt isoforms regulate breast cancer metastasis in a distinct manner and investigate the mechanistic basis for this selectivity. This application has three specific aims. Aim 1: I have recently identified an actin bundling protein, palladin, as an Akt1-specific substrate. I further demonstrated that phosphorylation of palladin plays a critical role in inhibition of breast cancer cell migration. Here I will examine the physiological relevance of palladin phosphorylation in metastatic dissemination using xenograft models, and evaluate the correlation of palladin phosphorylation and invasiveness of human carcinoma. Aim 2: I propose to identify novel Akt isoform-specific targets which regulate cell migration using an unbiased, proteomic approach. Aim 3: The role of subcellular localization of Akt isoforms in cell motility will be examined. Together, these studies will provide new mechanistic insights into the differential roles of Akt isoforms in breast cancer metastasis. They will also address for the first time novel mechanisms of isoform-specific Akt signaling in breast cancer progression in vivo. In addition, the results of these studies will reveal Akt isoform-specific substrates that contribute to cancer progression, which could point to new targets for anti-cancer therapeutics. The studies in this application will allow me to attain advanced training in tumor biology focusing on breast cancer, in a supportive and highly focused training environment with guidance from my mentor and advisory committee who are leading experts in the field. The practical skills and knowledge that I will gain during the mentored phase will provide me with the tools necessary for launching my independent career at an academic institution studying signaling mechanisms in breast cancer pathogenesis.